What to look for when developing a mini-payment system for multiple stores?

Question

Payment systems

Good afternoon
, we plan to use one merchant account for several online stores and conduct all transactions through it. Accordingly, the question arises of the competent organization of payments and mutual settlements, both at the level of program interaction and organizational. Protection against possible attacks and fraud. Online stores are different legal entities and they need independent accounting.
Advise where to start? What can you read or see in this area?
Are there ready-made platforms that implement such functionality?